One thing is certain: Doubt, which opens in February, written and directed by John Patrick Shanley, is a film about which audiences will disagree. But that is precisely its aim: it centres on a priest who may - or may not - have abused a schoolboy at St Nicholas school in the Bronx, and it is about the nature of doubt itself. Meryl Streep is the poisonous prosecutor Sister Aloysius (played somewhere between a wasp and a bat). But it is Viola Davis as Mrs Miller, mother of the schoolboy, whose performance eclipses all the competition.

Davis is only on screen for 11 minutes but to watch her is to understand an entire life. It is an object lesson in what an actor can convey in a short space of time. She seems to carry her family history in her face: the violence at home and her unswerving love for her child. And she looks dowdily embattled but unassailable in her shabby fawn overcoat. This is no glamorous part - her eyes fill and her nose runs in a chill wind. But her contribution to the film's argument is to accept doubt as if it were a certainty, one of life's givens. She is Mother Courage, and, where necessary, Mother Cowardice, because for her love counts more than justice. It is no surprise that she has already been nominated for a Golden Globe as best supporting actress and is tipped to win an Oscar.

But I had a moment's disorientation on meeting Viola Davis in an opulent London hotel because she is at such an exaggerated remove from her screen character - glossily groomed and dressed becomingly in black, with no jewellery. As a person, too, she is without ornament: poised, lively, industrious, determined to give her career her all. She is 43 and has known the value of hard work all her life (she has been rewarded for it, too, winning a Tony in 2001 for her role in August Wilson's King Hedley II). "I am not ashamed to say we grew up in abject poverty in a predominantly white community on Rhode Island. Our housing situation was not desirable - the plumbing did not work and we were infested with vermin. But it was a ripe ground for an artist to navigate emotionally." For Davis, acting was escape. She was driven by a determination not to grow up poor. She and her sister (now a teacher) had great fun playing "wealthy white women". She took on the bosomy types and her sister was "always Twiggy", which, Davis says with a fond laugh, "was a far stretch".

Acting is less of an escape now: "I do it because I don't know how to be anything else." But playing opposite Meryl Streep took some getting used to: "Thank God we had three and a half weeks' rehearsal so that I could get past the gawking phase, the recognition that this is Meryl Streep, an icon." The work steadied her: "It is a foundation, serves you, helps you overcome fear and nerves." She relies on her craft (she trained at Juilliard) and makes it sound as though, to act well, you need a touch of the playwright. "I had to find out who Mrs Miller was and create an inner life for her. And I had to find the need driving her." And what was that need? "The need to be a good mother." Davis is not a mother herself but was able to draw upon her memory of her own mother to inform her performance. What Davis most empathised with was Mrs Miller's ability to see that "your life doesn't always turn out as you want it to. Yet whatever it is, you work with it. You have to accept what you have to accept."

And what of doubt? Does she suffer from it? "Absolutely. Self-doubt, political and cultural doubt. But I hate the nebulous world of I-don't-know because it leaves me with not enough power." But in marriage, she says, "doubt is imperative. You have to understand you aren't always right." Her husband, actor Julius Tennon, may be pleased to hear this. Yet she goes on to stress that doubt must co-exist with faith and vision because, "What else is there but a dream? And you can't trade in your dreams." She has her eye, you can see, on her own dream and has been outspoken about the discrimination against black actors. She would love to play roles that 'people don't associate with me'. I joke that she may have spent too long working on a film set in a religious institution because when I ask what her next role will be, she does not reply. She closes her eyes, puts both hands together and prays.